The present invention, in some embodiments thereof, relates to material science and, more particularly, but not exclusively, to newly modified nanoclays, processes of producing same and elastomeric composites containing same.
Synthetic rubbers are typically made of artificial elastomers. An elastomer is a viscoelastic polymer, which generally exhibits low Young's modulus and high yield strain compared with other materials. Elastomers are typically amorphous polymers existing above their glass transition temperature, so that considerable segmental motion is possible. At ambient temperatures, rubbers are thus relatively soft (E of about 3 MPa) and deformable.
Elastomers are usually thermosetting polymers (or co-polymers), which require curing (vulcanization) for cross-linking the polymer chains. The elasticity is derived from the ability of the long chains to reconfigure themselves to distribute an applied stress. The covalent cross-linking ensures that the elastomer will return to its original configuration when the stress is removed. Elastomers can typically reversibly extend from 5% to 700%.
Synthetic elastomer is typically made by the polymerization of a variety of petroleum-based precursors called monomers. The most prevalent synthetic elastomers are styrene-butadiene rubbers (SBR) derived from the copolymerization of styrene and 1,3-butadiene. Other synthetic elastomers are prepared from isoprene (2-methyl-1,3-butadiene), chloroprene (2-chloro-1,3-butadiene), and isobutylene (methylpropene) with a small percentage of isoprene for cross-linking. These and other monomers can be mixed in various proportions to be copolymerized to produce products with a range of physical, mechanical, and chemical properties.
SBR rubber is mostly used in tires production, which accounts for about half of the world consumption, over 10 million tons per year, of synthetic rubber.
Synthetic rubbery materials often further include, in addition to a rubbery polymer or copolymer such as SBR copolymer, ingredients which may impart to the rubber certain desirable properties. The most commonly utilized ingredients are those that cause crosslinking reactions when the polymeric mix is cured (or vulcanized), which are usually consisting of sulfur and one or more “accelerators” (e.g., sulfenamides, thiurams or thiazoles), which make the sulfur cross-linking faster and more efficient.
Two other ingredients that play an important role in vulcanization chemistry are known as “activators,” and commonly include zinc oxide and stearic acid. These compounds react together and with accelerators to form a zinc-containing intermediate compounds, which plays a role in the formation of sulfur crosslinks.
Many other materials have been added to synthetic rubbers, mostly with the aim of hardening it or reducing its production cost. The most commonly practiced materials, which are referred to herein and in the art as fillers or reinforcing agents, include finely divided carbon black and/or finely divided silica.
Both carbon black and silica, when added to the polymeric mixture during rubber production, typically at a concentration of about 30 percent by volume, raise the elastic modulus of the rubber by a factor of two to three, and also confer remarkable toughness, especially resistance to abrasion, on otherwise weak materials such as SBR. If greater amounts of carbon black or silica particles are added, the modulus is further increased, but the strength may be lowered.
However, reinforcement of rubbers with carbon black or silica may disadvantageously result in rubbers characterized by lower springiness (resilience) and decreased stiffness after flexing. Elastomeric composites containing carbon black and silica are thus relatively brittle at low temperatures. Furthermore, the preparation of elastomeric composites containing CB or silica is difficult.
Studies have shown that for a filler to be reinforcing, the filler particles must have small diameter, at the nanometer range, for instance 10-50 nm, and must be well-adhered by the elastomer.
To this effect, studies have focused in recent years on the developments of hybrid (nanofiller-fibre) nanocomposites as an alternative to heavily filled elastomers. Such a nanofiller is typically made of nanoparticles, such as nanoclays, which are clays modified so as to obtain clay complexes that are compatible with organic monomers and polymers (also referred to herein and in the art as compatibilizers).
Nanoclays are easily compounded and thus present an attractive alternative to traditional compatibilizers. Nanoclays have been known to stabilize different crystalline phases of polymers, and to possess the ability of improving mechanical and thermal properties. For improved performance and compatibility, nanoclays are typically modified so as to be associated with organic moieties, and the modified nanoclays are often referred to as organomodified nanoclays. Organomodified nanoclays are typically prepared by treatment with organic salts. Negatively charged nanoclays (e.g., montmorillonites) are typically modified with cationic surfactants such as organic ammonium salts or organic phosphonium salts, and positively charged nanoclays (e.g., LDH) are typically modified by anionic surfactants such as carboxylates, sulfonates, etc. Exemplary organomodified montmorillonites are disclosed in Kim et al., Macromolecular Research, Vol. 17(10), pp. 776-785 (2009); in U.S. Pat. Nos. 6,818,693 and 6,407,155; and in WO 2005/113660.
The effect of nanoclays modified by hydrolysed mercaptosilane, as a substitute for carbon black, on the properties of SBR compounds, was reported at the 4th International Conference on nanotechnology for the plastics & rubber industries, http://www(dot)plastic(dot)org(dot)il/nano/nano_02_09_shenkar/PresNanolFeb_09_a damdotppt#2.
In short, it was reported that modified nanoclays may be produced by reacting nanoclays (NCs) such as organomodified montmorillonites (OMMT, e.g., Cloisite 30B), with mercaptosilanes. Such hybrids have been found useful in at least partially substituting for carbon black in elastomeric composites.
Additional background art includes Amit Das, Francis Reny Costa, Udo Wagenknecht, Gert Heinrich, European Polymer Journal 44 (2008) 3456-3465, available at www(dot)elsevier(dot)com/locate/europolj; Das, R. N. Mahaling, K. W. Stöckelhuber, G. Heinrich. Composites Science and Technology, Issue 71 (2011), Pages 276-281, available at www(dot)elsevier(dot)com/locate/compscitech; Yoong Ahm Kim, Takuaya Hayashi, Morinobu Endo, Yasuo Gotoh, Noriaki Wada, Junji Seiyama. Scripta Materialia, Issue 54 (2006), Pages 31-35, available at www(dot)sciencedirect(dot)com; and Xin Bai, Chaoying Wan, Yong Zhang, Yinghao Zhai. Carbon, Volume 49, Issue 5, April 2011, Pages 1608-1613, available at www(dot)elsevier(dot)com/locate/carbon.